Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of data tracks 6 defined by servo sectors 40-4N recorded around the circumference of each data track. Each servo sector 4i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 4i further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
FIG. 2 shows a conventional closed loop feed-back system for generating a control signal U 16 applied to the VCM 18 in order to seek the head to a target data track. The current position of the head y is compared to a reference position r in order to generate a position error signal (PES) 20. The PES 20 is filtered by a compensation filter C 22 to generate a feed-back control signal Ufb 24 which is added to a feed-forward control signal Uffwd 26 to generate the VCM control signal U 16. The feed-forward control signal Uffwd 26 is generated by filtering the reference signal r with an inverse filter 28 having an impulse response that models the inverse response of the VCM 18. If the inverse filter 28 models the inverse response of the VCM 18 exactly, then the feed-back control signal Ufb 24 will be zero. That is, the feed-forward signal Uffwd 26 will cause the VCM 18 to move the head to follow exactly the reference signal r so that the PES 20 is zero.
It is difficult to design the inverse filter 28 to exactly model the inverse of the VCM 18 due to design variations across VCMs, such as variations in the resonance profile and the torque factor Kt. In addition, certain VCM parameters may vary over time, for example, the torque factor Kt may vary with temperature fluctuations.